1. Field of the Invention
The present invention relates to an optical element and a scanning optical apparatus using the same. Particularly, the present invention is suitably applicable to image-forming apparatus, for example such as laser beam, printers or digital copiers employing an electrophotographic process, wherein a beam optically modulated by and emitted from a light source means, is deflected and reflected by an optical deflector comprised of a rotary polygon mirror etc. and thereafter the beam scans an area on a scanned surface through a scanning lens system (imaging optical system) having an f.theta. characteristic to record image information thereon.
2. Related Background Art
In the conventional scanning optical apparatus, such as the laser beam printers (LBP), image recording is carried out in such a way that a beam (laser beam), optically modulated according to image signals and emitted from the light source means, is regularly deflected by the optical deflector, for example comprised of a rotary polygon mirror (polygon mirror), and the beam is focused in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the imaging optical system having an f.theta. characteristic, and the beam scans the area on the surface.
FIG. 1 is a schematic diagram of the main portion of a conventional scanning optical apparatus.
In FIG. 1, a divergent beam emitted from light source means 71 is converted into a nearly parallel beam by collimator lens 72, and the beam (quantity of light) is limited by stop 73 to enter a cylindrical lens 74 having a predetermined refractive power only in the sub-scanning direction. The nearly parallel beam incident into the cylindrical lens 74 emerges in the as-incident state in the main-scanning plane. The beam is converged in the sub-scanning plane to be focused as a nearly linear image on a deflection facet 75a of optical deflector 75 comprised of a rotary polygon mirror (polygon mirror). Then the beam deflected and reflected by the deflection facet 75a of the optical deflector 75 passes through the scanning lens system (imaging optical system) 76 having an f.theta. characteristic to be guided onto a photosensitive drum surface 78 as a scanned surface, and the optical deflector 75 is rotated in the direction of arrow A to scan the area on the photosensitive drum surface 78 in the direction of arrow B, thereby performing recording of the image information.
Use of plastic a lens is mainstream for the scanning lens as an optical element used in the above scanning optical apparatus, because advanced aberration correction is possible by making its optical surfaces (lens surfaces) aspherical, because the lens can be manufactured at a low cost by injection molding, and so on. This plastic scanning lens, however, has larger manufacturing errors than glass scanning lenses. Because the, stability of injection molding is poor the plastic lens has the problem of producing a variation in spot diameters on the scanned surface, thus causing degradation of image quality.
Particularly, the peripheral portions in the longitudinal direction (in the main scanning direction) of the scanning lens have smaller lens thicknesses and thus pose such problems that:
(1) surface deformation is more likely to occur upon release from the mold; and PA1 (2) surface deformation and constriction is more likely to occur because the lens is cooled quickly, when compared with the central portion where the lens thicknesses are large. Therefore, a variation in the spot diameters prominently occurs in the peripheral portions of the image. PA1 when the X-axis is defined along the optical-axis direction of the optical element and the Y-axis along an axis perpendicular to the optical axis in the main-scanning plane, shapes in the main-scanning direction of the image-effective portions of the optical element are expressed by the following polynomial: EQU X={Y.sup.2 /R}/{1+(1-(1+K) (Y/R).sup.2).sup.1/2 }+B.sub.4 Y.sup.4 +B.sub.6 Y.sup.6 +B.sub.8 Y.sup.8 +B.sub.10 Y.sup.10 PA1 where R is a radius of curvature and K, B.sub.4, B.sub.6, B.sub.8, B.sub.10 are aspherical coefficients; PA1 when the X-axis is defined along the optical-axis direction of the resinous lens and the Y-axis along an axis perpendicular to the optical axis in the main scanning plane, shapes in the main-scanning direction of the image-effective portions of the resinous lens are expressed by the following polynomial: EQU X={Y.sup.2 /R}/{1+(1-(1+K)(Y/R).sup.2).sup.1/2 }+B.sub.4 Y.sup.4 +B.sub.6 Y.sup.6 +B.sub.8 Y.sup.8 +B.sub.10 Y.sup.10 PA1 where R is a radius of curvature and K, B.sub.4, B.sub.6, B.sub.8, B.sub.10 are aspherical coefficients;
Since this problem does not appear only in the portions of the minimum lens thickness but also affects regions around it, image-ineffective portions of the lens with the minimum thickness generally influence image-effective portions as well. This demands an increase in the thicknesses of the lens peripheral portions to some extent, which raises the cost because of an increase in the thickness of the whole lens and an increase in the molding-tact time.